1. Field of the Invention
This invention relates to improved techniques for press bending a malleable plate, such as a metal plate, by inserting a punch into a recess of a die. More particularly, the present invention relates to such press bending techniques for deforming the malleable plate into a shape conforming to a distal end face and a side face of the punch, thereby bending the plate into a desired shape.
2. Description of the Related Art
Japanese Laid-Open Patent Publication No. 3-264116 discloses a related conventional method of press bending a metal plate. This conventional method will be described with reference to FIGS. 7 and 8. Press 1 has been adapted for bending both ends of a steel plate W approximately at right angles to a central portion of the steel plate W, and thereby form a product generally having a U-shape in cross section. Such a press has been previously used to manufacture bumpers for automobiles.
The press 1 includes a die 4 fixed at a predetermined position and a vertically movable punch 2. FIG. 7 illustrates an example in which the bending operation of the steel plate W by the punch 2 and die 4 has been completed.
The punch 2 is formed into a thick plate-like shape and has, between bottom and side faces thereof, two corners or rounded edges that serve to bend the plate W. Each corner is chamfered into a rounded face 2r. The punch 2 has a distal end, namely, a bottom or terminated end formed with a shallow groove 2m for the plate W to permit metal to be transferred from the central portion of the plate to the bent corners during the bending process.
The die 4 has a recess 4m into which the punch 2 is inserted. Each opening edge between a top face 4u and a side face 4s of the recess 4m also is chamfered to a rounded face 4f. Each corner between the side face 4s and a bottom face 4b of the recess 4m also is chamfered to a rounded face 4r which forms the bent portion Wt of the metal plate W (see FIG. 8(C)) in co-operation with the punch 2. The clearance between the punch 2 and the recess 4m of the die 4 is set to be as small as possible to enhance bending accuracy.
In the press bending operation, the flat plate W is set on the top face 4u of the die 4, and the punch 2 is then moved downward. The punch 2 is inserted into the recess 4m of the die 4 (see FIG. 8(A)). In the process of the punch 2 moving to reach its lowest position (see FIG. 8(B)), the plate W is deformed into a shape conforming to the distal end face and side faces of the punch 2, so that both ends of the plate W are bent approximately at right angles to its central portion.
According to the above-described method, the clearance between the punch 2 and the recess 4m of the die 4 is set to be as small as possible, and the opening edge of the recess 4m is formed as the rounded face 4f. Accordingly, both ends of the plate W are brought into face-to-face contact with the rounded face 4f of the die 4 when the punch 2 is inserted into the recess 4m of the die 4 to bend the plate W (see FIG. 8(A)). Subsequent to the face-to-face contact with the rounded face 4f, the ends of the plate W are pressed into the clearance between the punch 2 and the recess 4m (see FIG. 8(B)). In other words, both ends of the plate W are rubbed against the rounded face 4f and thereafter pressed into the clearance. Therefore, each end of the plate W is pressed by the punch 2 and the die 4 with an unrelieved stress remaining in each end. The remaining stress tends to deform each bent edge to conform to the rounded face 4f of the die 4. This unrelieved stress results in local warping Ws or swelling Wh as shown in FIG. 8(D), when the product W is taken out of the press 1 after being bent.
Furthermore, the bent portion Wt of the plate W is closely pressed between the rounded face 2r of the punch 2 and the rounded face 4r of the recess 4m. Accordingly, a stress resulting from the difference in the length between the inside corner and the outside corner of the bent portion Wt of the plate W remains in the bent portion Wt. The remaining stress tends to stretch the inside corner of the bent portion Wt, resulting at the bent portion Wt as shown in FIG. 8(C). Thus, the above-described conventional press bending method does not accurately reproduce the desired shape for the bent products.